Organic materials can be used in optoelectronic applications for the processing of light, such as in electroluminescent devices (light emission) and solar cells (light absorption). Organic optoelectronic components are typically based on a layered structure of at least one light processing (or active) layer between two electrode layers. Light processing includes light emission, light absorption, modulation of light, wavelength conversion, and waveguiding. Various organic materials are optoelectronically active such that they can be used either to emit or to detect electromagnetic radiation. For example, organic optoelectronically active materials which can be used in the manufacture of Organic Light-Emitting Devices (OLEDs) include polymers and molecules in which the structure of molecular orbitals enables excitation of electrons to a higher excited state, which is thereafter discharged in the form of electromagnetic radiation. In absorbing devices, electromagnetic radiation generates an electric current in a circuit coupled to the electrodes of the device.
FIG. 1 depicts OLED 100, which includes substrate 102 with a layer of indium tin oxide as an anode 104, a layer of hole-transporting materials (HTL) 106, a layer of light processing material 108, such as emissive materials (EML) including emitter and host for an OLED, a layer of electron-transporting materials (ETL) 110, and a metal cathode layer 112. The emission color of an OLED is determined by the emission energy (optical energy gap) of the light processing material 108. Phosphorescent OLEDs (i.e., OLEDs with phosphorescent emitters) have a higher device efficiency that other OLEDs, including fluorescent OLEDs. Light emitting devices based on electrophosphorescent emitters are described in WO 00/70655 to Baldo et al., which is incorporated by reference herein.
Referring to FIG. 2, a typical organic solar cell (photovoltaic) device 200 includes substrate 202 with a layer of a transparent conductive electrode such as indium tin oxide (or other optically transparent conductive material) as an anode 204, a layer of donor-type materials 206, a layer of acceptor-type materials 208, a layer of exciton blocking materials 210, and a layer of metal cathode 212. U.S. Patent Publication No. 2002/0189666 to Forrest et al., which is incorporated by reference herein, describes examples of organic solar cells.
FIG. 3 depicts a schematic illustration of the process involved in the generation of photocurrent from incident light in a donor-acceptor (DA) heterojunction photovoltaic cell. Photon absorption occurs with an efficiency ηA proximate the anode 204. Exciton diffusion occurs in the donor-type material 206, where the fraction of excitons reaching the DA junction is ηED. A charge transfer reaction occurs proximate the acceptor-type material 208 with efficiency ηCT. Collection of carriers proximate the cathode 212 occurs, with an efficiency ηCC.
An efficient photovoltaic cell has a high photon absorption efficiency ηA, a high exciton diffusion efficiency ηED, a high charge transfer efficiency ηCT, and a high carrier collection efficiency ηCC. Absorbers with high exciton diffusion length, including certain organometallic complexes with heavy metals and triplet absorbers, provide a higher device efficiency than other known absorbers.